Demand for portable electronic devices is increasing each year. Example portable electronic devices include: laptop computers, personal data assistants (PDAs), cellular telephones, and electronic pagers. Portable electronic devices place high importance on total weight, size, and battery life for the devices.
Most portable electronic devices employ rechargeable batteries. Commonly used rechargeable batteries include Nickel-Cadmium (NiCad), Nickel-Metal-Hydride (NiMHi), Lithium-Ion (Li-Ion), and Lithium-Polymer based technologies. Charger circuits are commonly available for each of these types of battery technologies.
Example battery charger circuits typically have two operating modes: a constant current mode, and a constant voltage mode. In the constant current mode, a charging current (I) is delivered to the battery (or load) via a regulator that maintains a regulated charging current. As the voltage level of the battery approaches the desired final voltage, the circuit switches to a constant voltage operating mode. In the constant voltage mode, the output voltage is regulated while a trip-point detection circuit monitors the charging current that is delivered to the battery. The trip-point detection circuit initiates a trip point signal when the charging current indicates that the battery has reached a fully charged state (e.g., 10% of the maximum charging current in the constant voltage mode).
FIG. 6 is an illustration of a conventional trip-point detection circuit (600). Trip-point detection circuit 600 includes two amplifiers (AMP1, AMP2), a comparator (CMP), three resistors (R601, R601, and RSNS), a resistor ladder, and a transistor (MP). Amplifier AMP1 evaluates the voltage drop across resistors RSNS and R601, and includes a trim adjustment control (TRIM) for adjusting the offset in the amplifier. Transistor MP is responsive to the output of amplifier AMP1 such that the current through resistor R602 is responsive to changes in the output current level (IOUT). Amplifier AMP2 receives a reference voltage (VREF) and is arranged in a feedback loop with the resistor ladder to provide an adjustable comparison level (VTRIP). The resistor ladder is configured with trimming adjustments (TRIM LEVEL) to accommodate for process related inaccuracies in the ladder network, as well as a trip level selection (TRIP LEVEL) to change the comparison level (VTRIP). Comparator CMP compares VTRIP to the voltage across resistor R602, which is related to the output current level (IOUT), and asserts the trip signal when the output current level drops below a predetermined level set by VTRIP.